Conventional road-based navigation systems determine a suggested route based on a stored information base. The information base is usually a digital map containing passable roads. The suggested route may be communicated to a driver of the vehicle as a driving recommendation via a communications system. In those navigation systems, by way of a positioning of the vehicle a comparison is made between an ascertained position and the suggested route, and the driving recommendation is determined with reference to an area surrounding the vehicle and, in advance, with reference to the suggested route. Since the positioning system does not output an absolute position but merely outputs a position probability, it is necessary to extend the roads in the digital map by a tolerance range on both sides. If the position found lies within the tolerance range, it is assumed that the vehicle is on a road. This procedure, referred to as a matching procedure, outputs the current position of the vehicle as deviating from a road only when the tolerance range is exceeded. In that case, the method is unable to associate a road or its surrounding tolerance range with the current position and it defines the current position as off-road or open terrain. If the vehicle physically reaches and drives along a road when coming from an off-road area, the method requires a period of time for the matching procedure before the position of the vehicle on the road is recognized and before a situation-adapted driving recommendation may once again be output for the driver.
German Patent No. DE 101 46 115 B4 describes a method with which destination guidance to a final destination is carried out with the aid of directional navigation when a position of a vehicle is identified as being outside a known road network and no alternative known road is available for destination guidance.